In the practice of this art, the placement of the mask directly against the substrate causes considerable wear on the mask, and accordingly, a small separation distance has been tried. However, the light passing through the very narrow slits in the mask, as used in the best practice heretofore known, is still subject to an unfortunate degree of Fresnel diffraction, resulting in non-uniform printing of fine line geometries used in microcircuits. This effect is a disturbing problem even at separation distances of only 0.0002 to 0.0010 in.
It is accordingly the major purpose of the invention to virtually eliminate or annul Fresnel diffraction patterns in microcircuit printing.